boiler locking out

[paul evenden]

Hi all.

I have an interesting issue with a fairly new underfloor heating setup in a new extension.

I have a Grant oil fired system boiler running 20 radiators ,a 300 litre hot water tank (which also has solar hot water heating) and a two circuit underfloor system.

There are three programmers each switching a zone valve , one for each circuit.

After  a bit of checking I have discovered that if the underfloor circuit mixer pump starts recirculating water and not accepting water from the boiler, if the zone valves for the other two circuits happen to be closed then the hot water goes straight back to the boiler via the ABV valve and the boiler sees it as an overheat so trips out and has to be reset. 

The problem occurs infrequently, the issue seems to be caused by the UFH programmer calling for heat ( as the floor is not quite up to set temperature) via the zone valve but the heated water has nowhere to go ( because the UFH mixing valve won't accept the hot water).

Any thoughts please ?

How can I stop the programmer calling for heat even though the room isn't up to temperature when the mixing valve shuts off the hot water feed ?

If of course either of the other zone valves happen to be open then the lock out doesn't occur because the hot water has somewhere else to go. 

Is there a boiler fault ? Perhaps it should just detect the water coming back very hot and stop trying to heat it further ?

Paul   

[Temp]

7 minutes ago, paul evenden said:

After  a bit of checking I have discovered that if the underfloor circuit mixer pump starts recirculating water and not accepting water from the boiler, if the zone valves for the other two circuits happen to be closed then the hot water goes straight back to the boiler via the ABV valve and the boiler sees it as an overheat so trips out and has to be reset. 

 

We also have a Grant Oil fired boiler although ours heats a TS which feeds UFH so it behaves differently.

 

I don't think your boiler should shut down totally in the situation you describe. It should just cycle. Eg it should turn the flame off leaving the pump running until the flow temperature falls again. I wonder if the pump is turning off when it shouldnt? 

 

You could try turning down the dial on the boiler. This will make it cycle more but might avoid it getting so hot it locks up. Don't turn it down too much, you want it as high as possible without lockup occuring.

 

 

[dpmiller]

how do you know it's "locking out" and how are you re-setting it?

[Radian]

5 hours ago, paul evenden said:

then the hot water goes straight back to the boiler via the ABV valve and the boiler sees it as an overheat

 

Are you sure? If the valve is set correctly the boiler should just cycle off. If the valve isn't opening however, it could cause the boiler to overheat due to lack of flow.

[paul evenden]

I know it locks out as to restart it I have to unscrew the plastic cap over the reset button and press it.

I know the water is going through the ABV valve as I can hear it when its happening and alter the flow by adjusting the valve when its doing that.

Maybe there is a thermostat on the boiler that isn’t working that should be shutting the boiler off (without it tripping out) when the hot water is coming straight back to it ?

Maybe I shall phone Grant when they reopen in the new year. The underfloor heating people don’t understand what I’m telling them so their response was just to send me a load of wiring diagrams.

paul

[ProDave]

2 minutes ago, paul evenden said:

I know it locks out as to restart it I have to unscrew the plastic cap over the reset button and press it.

I know the water is going through the ABV valve as I can hear it when its happening and alter the flow by adjusting the valve when its doing that.

Maybe there is a thermostat on the boiler that isn’t working that should be shutting the boiler off (without it tripping out) when the hot water is coming straight back to it ?

Maybe I shall phone Grant when they reopen in the new year. The underfloor heating people don’t understand what I’m telling them so their response was just to send me a load of wiring diagrams.

paul

So this is the overheat thermostat tripping, not the burner lockout that is usually referred to as lockout.

 

Assuming this over heat thermostat therefore is on the return pipe, and the problem only happens when nothing in the system is actually taking any heat out, so the return temp is pretty much the same as the flow temp, then I would suggest the boiler flow temp is too high.

 

Since you can't adjust the over heat thermostat trip temperature I would reduce the flow temperature from the boiler so that the flow temperature is comfortably lower that the over heat trip temperature.

[paul evenden]

Thank you Prodave

someone else suggested that yesterday too.

I turned it down a little last night and so far it hasn’t tripped out again but as it only happens occasionally it may be to early to tell.

As we have such a big system for the boiler what would the implications be of me having its output raised next time its serviced ? Is an 18/25 system boiler and is currently running at 23kw ,I believe uprating it to its full 25kw is just a nozzle change and an adjustment ?

Or would that make this current issue worse ?

paul

[ProDave]

It is the output temperature you want to turn down, not the boiler power.

[paul evenden]

Yes I understand that

I was thinking though since I recently added the underfloor heating in my new kitchen extension and two extra radiators elsewhere, that a bit more output might be needed . According to our energy performance certificate the house is just over 300 sq metres (which I doubt and intend to check) It hasn’t been really cold yet this year though but it seems to be coping fine at present.

When I had the boiler fitted four years ago I went on the principle that a smaller one working hard would be more efficient than the next size up not really being pushed, its only recently that I found that the one I have is only set up on its mid power setting.

paul

Edited December 28, 2021 by paul evenden

[dpmiller]

isn't there a temperature dial on the boiler beside the overtemp reset cap?

[Temp]

3 hours ago, paul evenden said:

Is an 18/25 system boiler and is currently running at 23kw ,I believe uprating it to its full 25kw is just a nozzle change and an adjustment ?

Or would that make this current issue worse ?

Sorry if you know this but..

 

Modulating Gas boilers can turn the flame power up and down to match demand when there are only small loads (eg just one UFH loop calling for heat). I don't think any consumer grade oil boilers can do this. The flame is either fully ON or OFF. 

 

The "temperature dial" on an oil boiler (at least on my Grant) appears to set the maximum flow temperature during cycling. So when there is a small load what happens is...

 

1) The load stat calls for heat. 

2) Boiler fires up the burner and pump.

3) Output and return temperatures start rising.

4) When the flow reaches the temperature set on the dial it (should) switch off the flame but leave the pump running.

5) Flow temperature starts falling.

6) At some point the boiler decides to fire up the flame again. Go to 3).

7) Only when the all loads are satisfied does the boiler turn itself and the pump off.

 

If you monitor the flow temperature over time you can see it rising and falling as it cycles around 3-6. Ideally these cycles should not be too fast because that's like driving a car in stop-start traffic. 

 

On the other hand If/when you have a heavy load it is possible the burner may run for long periods without the temperature ever quite reaching that set on the boiler dial. In this situation the boiler fires up when called by the load and keeps burning until the load is satisfied (eg no cycling). This is the most efficient mode but it can take some work to make it operate like this. The load has to be big enough, the pump speed just right etc.

 

So something is causing your boiler to overheat when its cycling. Ideally get an engineer to check it really is overheating not a faulty over temperature sensor.  I would check that when its cycling, and the flame goes off, the pump continues to run? Perhaps the pump is having problems starting when only the bypass is open? Could be tricky to diagnose.

 

Turning down the dial on the boiler may help but its not ideal because it makes cycling more likely. More likely to occur when it need not.

 

To answer the question above. I think increasing the burner power would most likely make things worse. It would reach the dial temperature sooner making cycling faster. Perhaps increase oportunities to overheat? We actually went the other way and fitted smaller jets which Grant say improves efficiency a few%. This allowed me to eliminate cycling when feeding our thermal store but that's a long story that wouldn't apply to your set up. I wouldn't change the jets yet. 

 

[paul evenden]

Thanks for that Temp

very helpful.

The engineer always sets the temperature knob on the boiler to max when he services it.

There has only been an issue since the underfloor circuit has been added, its taken off of the feed and return to the boiler before any of the radiators and from the boiler to the underfloor recirculation pump is only about three metres of 22mm pipe. What is happening is that when the recirculation pump for the underfloor circuits goes into recirculation mode and stops accepting hot water from the boiler ,because the underfloor programmer is still calling for heat via its zone valve the hot water from the boiler leaves, goes three metres to the underfloor pump then goes straight back at the same temperature and the boiler overheats and shuts off. If either of the other zone valves happen to be open then the water can go that way instead and so the boiler is ok.

What perhaps I need is to wire a thermostat of some sort on the return pipe  ,in the wire between the zone valve and the boiler to shut the boiler off when the underfloor pump won’t accept water from the boiler.

paul

Edited December 28, 2021 by paul evenden
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[dpmiller]

turn the temp down, how hot do you need your hot water to be???

[Temp]

11 minutes ago, paul evenden said:

What is happening is that when the recirculation pump for the underfloor circuits goes into recirculation mode and stops accepting hot water from the boiler ,because the underfloor programmer is still calling for heat via its zone valve the hot water from the boiler leaves, goes three metres to the underfloor pump then goes straight back at the same temperature and the boiler overheats and shuts off.

 

That's almost correct behaviour.  When that happens the boiler is meant to temporarily shut off the burner to prevent overheating. 

  

15 minutes ago, paul evenden said:

What perhaps I need is to wire a thermostat of some sort on the return pipe  ,in the wire between the zone valve and the boiler to shut the boiler off when the underfloor pump won’t accept water from the boiler.

 

But the boiler should already be doing that but in a different way. When the bypass opens the boiler return will get hotter, that will in turn cause the flow to get hotter. When it exceeds the max temperature set by the dial the boiler should turn its flame off temporarily. Instead for some reason its not doing that, it appears to be going into overheat and shuting down requiring a reset. The question is why is the normal mechanism not always working. 

 

[dpmiller]

because it's set too high (is max 85?) and overshoot is hitting the limit stat (92 IIRC)

[paul evenden]

Thank you all.

I need to check the temperature on the return then I think in case there is a duff sensor on the boiler.

I have some temperature probes in my garage somewhere, I’ll have a look tomorrow.

paul

 

[paul evenden]

The water tank temperature is set at 65c (the max) and I have a blending valve on the outlet to drop it to 45c and another under the sink in the loo the grandkids use dropping that tap further so that they can’t scald themselves washing their hands.

There is a separate control system on the tank for the solar hot water, the boiler only heats the water if the solar hasn’t done so during the day, a programmer turns the boiler on for up to an hour at 8pm with an over ride for when we have a house full of visitors. The water tank is 300 litres. That side of it all works fine. 
paul

[dpmiller]

Show us a picture of the boiler.

[ProDave]

2 hours ago, paul evenden said:

The engineer always sets the temperature knob on the boiler to max when he services it.

He does that because he wants to use the flue gas analyser to set up the burner and he wants to ensure the boiler keeps on burning and does not shut off, so turning the t'stat up will give it a longer burn and time to do his adjustments.

 

It does not mean it should be left that high and really he should set it back where it was before he came to do the service.

[paul evenden]

Thanks Prodave

I’ve just been going through the boiler installation manual.

It shows a pipe thermostat as ‘optional’ depending on installation,  in the wiring diagrams.

We don’t have one.

I expect if we did that would fix this problem.

so I shall get one

the wiring is actually very simple.

paul

[dpmiller]

...photo please. The boiler's own stat is normally a dual-stat and there should be a knob beside the black cap. what is it set to?

[paul evenden]

Less than max now as following another posters advice yesterday I’ve turned it down a touch.

Prior to that it has been on max for the last four years with no problem until we connected the underfloor system up.

So far since turning it down the boiler hasn’t tripped out.

The boiler has two thermostats in pockets on the side of the housing.

There is no stat on either of the pipes.

I’ll measure some temperatures tomorrow.

paul

Edited December 28, 2021 by paul evenden
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